Electrical contact



y 6, 1941- c. w. KEITEL 2,241,262

ELECTRICAL CONTACT Filed Oct. 26, 1939 QY T BYW J TORNEY Patented May 6,1941 2,241,262 ELECTRICAL CONTACT Christian William Keitel, East Orange,N. 3., as-

signor to Baker & Company, Inc., Newark, N. 3., a corporation of NewJersey Application October 26, 1939, Serial No. 301,314

7 Claims.

This invention relates to electrical contact elements suitable for usein making and breaking of electric current, and is directed primarily tothe improvement of electrical contacts consisting of silver and silveralloys.

Electrical contacts should have high current carrying capacity and lowsurface contact resistance. They should be resistant to oxidation andhave great mechanical strength. In operation they should be subject toas little metal transfer as possible so as to avoid pitting andsticking. High melting point also is desirable as it decreases thedeleterious consequences of arcing.

-Electrical contacts are usually manufactured of platinum, alloys ofplatinum, tungsten or silver. Of these metals, fine silver has found thewidest use, as it is relatively cheap, has excellent current carryingcapacity and does not oxidize easily. Silver contacts, however, aresubject to the disadvantage that their contact resistance may increasein use, for instance in the presence of sulphurous gases and that theytend to stick particularly when the current used is too high.Furthermore, such contacts are deficient in hardness.

Various attempts have been made to overcome the disadvantages of silvercontact elements by incorporating metal carbides, cadmium, magnesium,gold, palladium, platinum and others, but no thorough success has beenobtained by any of such additions. The addition of base metals increasesthe tendency to build up surface resistance and usually decreases themelting point. The admixture of precious metals is usually benei'lcial,but attempts heretofore made to incorporate precious metals have notsucceeded in eliminating the shortcomings of silver contacts to asatisfactory degree.

A prime object of my invention is to provide a novel and improvedelectrical contact element that shall be hard, shall have high currentcarrying capacity, low surface contact resistance. high melting point,resistance to oxidation, 8. minimum tendency to pitting and sticking andgreat mechanical strength.

Other objects are to provide such an electrical contact that shall befree from the objections to the known silver contact elements but shallpossess the desirable characteristics of silver contacts; and to obtainother advantages and results as will be brought out by the followingdescription.

I have now found that the properties of silver contacts can be improvedconsiderably by the incorporation therein of refractory metals of theplatinum group, comprising the metals ruthenium, rhodium, iridium andosmium, particularly ruthenium. These metals do not form completelyhomogeneous solid solutions with silver and for this reason contacts ofsilver combined with such metals can be provided by melting only whensmall percentages of the refractory metals above mentioned'areincorporated, such as up to about 10%. Contacts of silver combined withlarge percentages of ruthenium or other refractory platinum groupmetals, therefore, are manufactured by powder metallurgical methods.

The admixture of even small amounts of such refractory metals, forinstance, 0.5% ruthenium, has been found to increasethe hardness and'melting point of silver contacts and to decrease sticking. Electricalcontacts according to my invention may comprise small amounts of. one ormore of the refractory platinum group metals, for instance ruthenium,either alone or in combination with another one or more of such metalsas rhodium, osmium and iridium, in amounts up to 10% or 15%, or they maycontain larger amounts of such metals, for instance up to over In allcases the crystal structure of the alloys is finer than that of puresilver. In cases where the refractory platinum group metal content ishigh, such as 90% or the construction material for my contacts is ofcourse produced by powder metallurgical methods, the silver servingessentially as a binder for the refractory platinum group metal ormetals particles.

Electrical contacts according to my invention have a great variety ofuseful applications. They have high current carrying capacity and lowsurface contact resistance. They have greater hardness, greatermechanical strength, and higher melting point than silver contacts, andhave been found to resist deformation and sticking to a considerabledegree.

My invention is equally applicable to electrical contacts of palladiumor alloys of silver such as alloys of silver and palladium. In the caseof electrical contacts of an alloy of palladium and silver in equalamounts theaddition of for instance l% ruthenium increases the RockwellB. hardness from 44 to 62 when cast, from 93 to 95 when hard, and from51 to 63 when annealed at 800 C. for 30 minutes.

The crystal structure of my contacts isfiner, pitting and stickingdecreased, and hardness, mechanical strength and melting point areincreased. Decrease of crystal size in pronounced degree was observedeven with admixtures of as little as 0.1% ruthenium.

Electrical contacts are often manufactured of composite metal in whichthe contact element itself comprises a thin eifective surface of theelectrical contact metal hacked or supported by base metals usually oflower cost than the metal of the electrical contact. Tungsten contactshave thus been backed by copper in United States Patent No. 1,089,907,by steel or iron in United States Patent No. 1,181,742, or by nickel andnickel alloys in United States Patent No. 1,155,- 426; precious metalcontacts have similarly been backed by less expensive metals, such assilver or platinum contacts backed by copper in German Patent No.203,904, gold or platinum contacts backed by nickel or steel as inUnited States Patent No. 1,469,043, or silver contacts backed by Monelor stainless steel as for instance described in United States Patent No.1,925,856. It should be understood, therefore, that my electricalcontacts may also be manufactured from composite metal so that theefiective contact surface consists of the alloys described by me andbacked, in accordance with old well-known practice, by anotherdissimilar metal, such as nickel, Monel, iron, steel, stainless steel,and so forth.

For the purpose of illustrating possible preferred embodiments of myinvention, I have shown two forms of contacts in the accompanyingdrawing in which Figure 1 is a fragmentary side elevational view of athermostatic switch having one form of contact element attached thereto,and

Figure 2 is a perspective View of another type of contact elementembodying my invention.

As above indicated, my invention may be embodied in various structuralforms of electric contact elements for many different types ofelectrical apparatus, and the specific embodiments herein illustratedshould be understood as being only as examples of two possible types ofcontact elements. a

In Figure 1, the contact element l is shown as having its shank rivetedor otherwise suitably secured to a bimetallic thermostatic strip 2.

In Figure 2 I have shown the contact element as comprising a base orbacking 3 of a base metal or base metal alloy, and a facing disc orcontact surface 4 formed of the alloy of my invention. The facing disc 4may be secured to the backing or body 3 in any suitable manner as bywelding or soldering.

Having thus described my invention, what I claim is:

1. An electrical contact element formed of a metal taken from the groupcomprising silver and palladium, combined with at least one metal takenfrom the group comprising ruthenium, rhodium, iridium and osmium.

2. An electrical contact element formed of silver combined withruthenium.

3. An electrical contact element formed of silver combined withruthenium, the ruthenium constituting from 0.1% to of the whole and theremainder being silver.

4. An electrical contact element formed of an alloy of silver andruthenium containing up to 15% ruthenium, the remainder being silver.

5. An electrical contact element formed of silver combined withruthenium, the ruthenium constituting the major proportion, and theremainder being silver.

6. An electrical contact element formed of an alloy of silver, palladiumand ruthenium.

7. An electrical contact element formed of an alloy of silver combinedwith ruthenium.

CHRISTIAN WILLIAM KEITEL.

